Co-rotating multi-shaft screw extruders have been known for a long time. A comprehensive over-view of such screw extruders is given in publication [1]=Kohlgrüber: The Co-Rotating Double-Screw Extruder. Hamer Publishing, Munich, 2007. Modern screw extruders have a modular system in which the various screw elements can be mounted on a core shaft. This allows the person skilled in the art to adapt the screw extruder to the process task in question. In this case, conveying elements, kneading elements, and/or mixing elements can be used and combined with one another.
For example, DE 102007055764 A1 discloses a screw for a twin-screw extruder having a screw main body comprising a combination of a plurality of segments with one another in the axial direction. The screw main body has a plurality of sections in the axial direction that perform various functions: a conveying section for conveying the material to be kneaded, a kneading section for kneading the material to be kneaded, and an extruding section for pressurizing and extruding the material to be kneaded.
One of the essential tasks carried out on multi-shaft screw extruders is the dispersion of liquid phases or liquid additives in polymer melts that cannot be homogeneously mixed with one another, or the dispersion of solids in polymer melts.
A further essential task carried out on multi-shaft screw extruders is the dispersion of a gaseous phase in a polymer melt. In a so-called degassing extruder, a gaseous phase serves as an entraining agent in a degassing zone downstream of the dispersion zone. The entraining agent causes (improved) foaming of the polymer melt, with a correspondingly larger free degassing surface area on the one hand and a partial reduction in pressure of the volatile substances to be degassed (monomers, oligomers, solvents) on the other. Examples of known entraining agents for this purpose are water, carbon dioxide, or nitrogen.
The quality of the dispersion has a multifaceted effect on the operating behavior of a screw extruder. Poor dispersion of gaseous, liquid, or solid additives reduces product quality as a result of inhomogeneities in the plastic masses. Poor dispersion of an entraining agent in the dispersion zone also causes degassing performance in a degassing zone downstream of the degassing zone to decrease, thus potentially making it impossible to achieve the desired residual concentrations of monomers, oligomers and solvents in the plastic mass.
Poor dispersion can be counteracted by increasing the rotational speed or decreasing the throughput. The drawback of both these measures is that this causes the temperature of the plastic mass processed in the screw extruder to increase, thus decreasing product quality due to increased damage kinetics.
DE 4134026 A1 discloses mixing elements that are suitable for dispersing fillers and liquid additives in polymer melts. The geometry of these mixing elements is based on a reverse-conveying, single-thread conveying element with an Erdmenger profile and have active-conveying grooves in their comb rows. Such mixing elements are known to the person skilled in the art as toothed mixing elements (TME).
DE 4134026 A1 discloses toothed mixing elements in which the pitch of the base geometry is 0.2 to 0.35 times the outer diameter of the toothed mixing element, with pitch being understood to denote the axial length required for a complete rotation of the screw profile. The pitch of the grooves is given as 2.5-6 times the outer diameter of the toothed mixing element.
JP 2001310369 A discloses toothed mixing elements in which the pitch of the base geometry is 0.1 to 0.5 times the outer diameter of the toothed mixing element.
JP 2012 213996 A discloses toothed mixing elements in which the pitch of the base geometry is 0.1 to 0.3 times the outer diameter of the toothed mixing element.
WO 2009/051279 A1 discloses mixing elements in which the pitch of the base geometry is 0.50 to 1.50 times the outer diameter. The mixing elements have 10 to 30 grooves per base geometry period.
However, it was observed in dispersion of a gaseous phase with a toothed mixing element according to DE 4134026 A1 that the bubbles formed are not homogenous in size, and that in particular, larger bubbles regularly occur. Large bubbles are detrimental in that their surface area-volume ratio is extremely small, with the result that a downstream degassing zone is provided with only a limited surface area for degassing. Moreover, it was observed that on addition of a gaseous phase to a polymer melt, in the area of the toothed mixing elements, only an axial length corresponding to the outer diameter of the toothed mixing elements is required in order to pre-disperse the added gaseous phase before the actual dispersion of the gaseous phase into more or less small bubbles begins.